Containers aren’t tightly coupled to the name Docker. You can use other tools to run containers
You can be running containers with Docker, or a bunch of other tools which aren’t Docker.
docker is just one of the many options, and Docker (the company) creates some of the awesome tools in the ecosystem, but not all.
There are two big standards around containers:
- Open Container Initiative (OCI): a set of standards for containers, describing the image format, runtime, and distribution.
- Container Runtime Interface (CRI) in Kubernetes: An API that allows you to use different container runtimes in Kubernetes.
How the Docker stack works
Docker Engine comes with a bunch of tools to make it easy to build and run containers as a developer, or a systems administrator. It is basically a command-line interface (CLI) for working with containers.
So, in reality, when you run a container with
docker, you’re actually running it through the Docker daemon, which calls containerd, which then uses runc.
docker command is just one piece of the puzzle. It actually calls down to some lower-level tools to do the heavy lifting:
What are the lower-level tools in the Docker stack?
From the bottom up, these are the tools that
docker uses to run containers:
- Lowest-level 🔩 The low-level container runtime. runc is a low-level container runtime. It uses the native features of Linux to create and run containers. It follows the OCI standard, and it includes libcontainer, a Go library for creating containers.
- 🔧 The high-level container runtime. containerd sits above the low-level runtime, and adds a bunch of features, like transferring images, storage, and networking. It also fully supports the OCI spec.
- 👺 The Docker daemon. dockerd is a daemon process (a long-running process that stays running in the background) which provides a standard API, and talks to the container runtime
- Highest level 👩💻 The Docker CLI tool. Finally, docker-cli gives you the power to interact with the Docker daemon using
docker ...commands. This lets you control containers without needing to understand the lower levels.
Does Kubernetes use Docker?
A really common question is “how do containers run in Kubernetes?”. Does Kubernetes use Docker? Well, it doesn’t anymore — but it used to.
Originally, Kubernetes used Docker (Docker Engine) to run containers.
But, over time, Kubernetes evolved into a container-agnostic platform. The Container Runtime Interface (CRI) API was created in Kubernetes, which allows different container runtimes to be plugged into it.
Docker Engine, being a project older than Kubernetes, doesn’t implement CRI. So to help with the transition, the Kubernetes project included a component called dockershim, which allowed Kubernetes to run containers with the Docker runtime.
It bridged the gap between the old world and the new.
Death of the shim also
But, as of Kubernetes 1.24, the dockershim component was removed completely, and Kubernetes no longer supports Docker as a container runtime. Instead, you need to choose a container runtime that implements CRI.
This doesn’t mean that Kubernetes can’t run so-called Docker-formatted containers. Both containerd and CRI-O can run Docker-formatted and OCI-formatted images in Kubernetes; they can do it without having to use the
docker command or the Docker daemon.
Open Container Initiative (OCI) specifications
The OCI was one of the first efforts at creating some standards for the container world. It was established in 2015 by Docker and others.
The OCI is backed by a bunch of tech companies and maintains a specification for the container image format, and how containers should be run.
For example: you might use one OCI-compliant runtime for your Linux hosts, but a different runtime for your Windows hosts.
Kubernetes Container Runtime Interface
The other standard we need to talk about is the Container Runtime Interface (CRI). This is an API that was created by the Kubernetes project.
CRI is an interface used by Kubernetes to control the different runtimes that create and manage containers.
So if you prefer to use containerd to run your containers in Kubernetes, you can! Or, if you prefer to use CRI-O, then you can. This is because both of these runtimes implement the CRI specification.
But, if you pay to get support (security, bug fixes etc) from a vendor, your choice of container runtime might be made for you. For example, Red Hat’s OpenShift uses CRI-O, and offers support for it. Docker provides support for their own containerd.
containerd and CRI-O
We’ve seen that Docker Engine calls down to a bunch of lower-level tools. But what are these tools? And how do they fit together?
The first layer is the high-level runtimes: containerd, created by Docker, and CRI-O, created by Red Hat.
containerd is a high-level container runtime that came from Docker. It implements the CRI spec. It pulls images from registries, manages them and then hands over to a lower-level runtime, which uses the features of the Linux kernel to create processes we call ‘containers’.
CRI-O is another high-level container runtime which implements the Kubernetes Container Runtime Interface (CRI). It’s an alternative to containerd. It pulls container images from registries, manages them on disk, and launches a lower-level runtime to run container processes.
Yes, CRI-O is another container runtime. It was born out of Red Hat, IBM, Intel, SUSE .
runc and other low-level runtimes
runc is an OCI-compatible container runtime. It implements the OCI specification and runs the container processes.
runc is sometimes called the “reference implementation” of OCI.
Other low-level runtimes
But, runc isn’t the only low-level runtime. The OCI specification is allowing other tools to implement the same functionality in a different way:
- crun a container runtime written in C (by contrast, runc is written in Go.)
- firecracker-containerd from AWS, which implements the OCI specification as individual lightweight VMs (and it is also the same technology which powers AWS Lambda)
- gVisor from Google, which creates containers that have their own kernel. It implements OCI in its runtime called
There is a set of open standards which, theoretically, make it easier to swap out different implementations. Projects like containerd, runc and CRI-O implement parts of those standards.
In Kubernetes, you can choose which container runtime you want to use, as long as it supports the CRI API. You can use containerd or CRI-O.
Finally Moral of the story is :
After k8s 1.24 version docker/dockershim has been depricated i.e. container can’t be managed by docker command
but u can use other container run time which supports CRI (K8s native API)
by-using crictl commands (or other supported commands) to manage containers/pods instead of docker commands.
Map docker cli over crictl : https://medium.com/@vineetcic/mapping-from-dockercli-to-crictl-life-after-docker-is-cri-a39ea5649d6c